Cerebrovascular disease, most notably brain ischemia, is a leading cause of death and long-term disability in the US. The current and only treatment is prompt restoration of blood flow to the ischemic tissue. However, a substantial portion of the damage caused by ischemia/reperfusion occurs during the reperfusion phase: as ischemic tissue is reoxygenated reactive oxygen species (ROS) are quickly generated, starting early during reflow. Reperfusion injury has proved difficult to treat pharmacologically, likely because effective drug concentrations have not built up sufficiently during the early phase of reperfusion. The mitochondrial electron transport chain (ETC) is a major site of ROS production during cellular stress due to ETC hyper-activation, which causes high mitochondrial membrane potentials (??m), which in turn trigger excessive ROS production. We thus propose that the ideal therapy should target the ETC non-invasively to prevent the generation of ROS from the onset of reflow. Accordingly, our overall goal in this application is to develop a new, non-invasive therapy to normalize mitochondrial hyperactivity during reflow. We will capitalize on the photoreceptive properties of cytochrome c oxidase (COX) for infrared light (IRL) to modulate mitochondrial activity, thereby attenuating the production of ROS and, as a result, limit ischemia/reperfusion injury in the brain. Cytochrome c oxidase is the primary cellular photo-acceptor of IRL and the terminal enzyme of the ETC. We have discovered four specific IRL wavelengths that partially inhibit COX (instead of activating COX, i.e., the current paradigm). We show that inhibitory IRL, applied at the time of reperfusion, provides profound neuroprotection. In this proposal, we will build on these compelling preliminary data and capitalize on the unique, multi-disciplinary expertise of our research team to: Identify the combinations and energies of our four IRL wavelengths that yield optimal inhibition of COX and mitochondria in vitro using isolated rat brain COX and mitochondria (Aim 1). Investigate the mechanism of IRL-mediated protection in support of our central hypothesis of IRL action during reperfusion: IRL ? COX activity? ? ??m? ? ROS? ? viability?, using real-time imaging of mitochondrial funtion in rat primary neural cells exposed to simulated ischemia-reperfusion (Aim 2). Develop IRL-mediated protection and identify the optimal temporal treatment paradigm using a rat model for global brain ischemia to maximize post-ischemic neuroprotection (Aim 3).